The present invention relates to a process and an apparatus for the removal of nitrous oxide from a feed gas stream.
The cryogenic purification of air requires a pre-purification step for the removal of high-boiling and hazardous materials. Principal high-boiling air components include water and carbon dioxide. If removal of these impurities from ambient air is not achieved, then water and carbon dioxide will freeze out in cold sections of the separation apparatus (for example in heat exchangers and liquid oxygen sump) causing pressure drop and flow and operational problems. Various hazardous materials including acetylene and other hydrocarbons also need to be removed. High-boiling hydrocarbons are a problem because they concentrate in the liquid oxygen section of the separation apparatus, resulting in a potential explosive hazard.
A minor air component that has recently been recognised as important in the pre-purification of air is nitrous oxide. Nitrous oxide is present in ambient air at concentrations of about 0.3 ppm. Nitrous oxide has similar properties to carbon dioxide and therefore presents potential operation problems resulting from solids formation in the separation apparatus column and heat exchangers. In addition, nitrous oxide presents a safety hazard as it is known to enhance combustion of organic materials, and it is shock sensitive. Therefore, there is significant industrial interest in the removal of trace nitrous oxide from ambient air prior to cryogenic distillation.
The nitrous oxide concentration in the atmosphere is steadily increasing at about 0.2 to 0.3% a year as a result of exhaust-gases of sewage treatment plants and catalysts of combustion engines and thermal plants. In addition, attention to nitrous oxide has increased as the required purity of gaseous products (for example rare gases and oxygen) has increased. Many previous solids formation problems noted in air separation apparatus and attributed to carbon dioxide may have been due to nitrous oxide.
Current technology for the pre-purification of air consists of adsorptive treatment including thermal swing (TSA) adsorption processes (disclosed for example in U.S. Pat. No. 4,541,851 and U.S. Pat. No. 5,137,548) and pressure swing adsorption (PSA) processes (disclosed for example in U.S. Pat. No. 5,232,474). In general such systems are designed for total water and carbon dioxide removal. A recent publication (Wenning, MUST meeting, 1996) highlighted the problem of nitrous oxide in air separation plants.
Wenning teaches that in conventional TSA processes using zeolites such as 5A, nitrous oxide is less strongly adsorbed than carbon dioxide. This leads to breakthrough of nitrous oxide before carbon dioxide. Nitrous oxide then enters the cold section of the apparatus. Similar results occur in alumina-based PSA processes. The applicants have shown that an all-alumina PSA process removes about 30% of the inlet nitrous oxide concentration in air. Mixed beds of alumina and zeolite used in TSA or PSA processes are known to result in more nitrous oxide breakthrough than non-mixed beds.
Catalysts are available which convert nitrous oxide to nitrogen and oxygen (Wenning), but these catalysts function at elevated temperatures which is undesirable.
U.S. Pat. No. 5,919,286 teaches using a layer of zeolite at the product end of a PSA bed for nitrous oxide removal. U.S. Pat. No. 6,106,593 teaches a 3 layer TSA adsorbent bed where the final adsorbent layer removes nitrous oxide. The adsorbent is defined by a minimum adsorption capacity for nitrous oxide, and includes adsorbents such as CaX.
US 6,273,939 teaches the use of faujasite type zeolites, especially calcium-exchanged X or LSX, for the removal of nitrous oxide from air prior to cryogenic distillation. The adsorbent is defined by size only.
EP-A-1064978 teaches the use of barium-exchanged zeolite for removal of carbon dioxide, nitrous oxide and organic impurities from air prior to cryogenic distillation. 13X zeolite is used, which is stated to show a large amount of nitrous oxide breakthrough before carbon dioxide breakthrough.
EP-A-1092465 teaches the use of X-type zeolites with specified silicon to aluminum ratios for adsorptive removal of nitrous oxide and hydrocarbons from air prior to cryogenic distillation. Siliporite G586 from Ceca in 1.6xc3x972.5 mm beads is used.
In these documents, adsorbents useful for nitrous oxide removal from air are identified by their composition or equilibrium selectivity.
In a first aspect, the present invention provides a process for removing water, carbon dioxide and nitrous oxide from a feed gas stream, comprising passing the feed gas stream through a first adsorbent to adsorb water, a second adsorbent to adsorb carbon dioxide and a third adsorbent to adsorb nitrous oxide and to form a purified feed gas stream, wherein the third adsorbent has a nitrogen diffusion parameter of 0.12 secxe2x88x921 or higher and a nitrous oxide capacity of 79 mmol/g/atm or higher at 30xc2x0 C. and the first, second and third adsorbents may optionally be the same material.
Preferably, the third adsorbent has a nitrogen diffusion parameter of 0.15 secxe2x88x921 or higher.
Optionally, the second and third adsorbents are the same material and are different from the first adsorbent.
Preferably, the feed gas is air.
Preferably, the process further comprises cryogenic distillation of the purified feed gas stream to separate a nitrogen rich stream and/or an oxygen rich stream.
Preferably, the adsorbents are regenerated by thermal swing adsorption. The adsorbents are preferably regenerated at a temperature of 50 to 400xc2x0 C. and/or at a pressure of 0.1 to 20 atm. Preferably, oxygen, nitrogen, methane, hydrogen, argon or a mixture of two or more thereof is passed over the adsorbents as they are regenerated.
Preferably, the feed gas stream is at a temperature of 0 to 50xc2x0 C., and/or at a pressure of 3 to 20 atm.
The first adsorbent may be selected from alumina, silica gel, impregnated alumina, zeolite A and zeolite X, and the second adsorbent may be selected from impregnated alumina, impregnated composite alumina/zeolite, zeolite A and zeolite X.
In a second aspect, the present invention relates to apparatus for removing water, carbon dioxide and nitrous oxide from a feed gas stream comprising in fluid series connection a first adsorbent to adsorb water, a second adsorbent to remove carbon dioxide and a third adsorbent to remove nitrous oxide, wherein the third adsorbent has a nitrogen diffusion parameter of 0.12 secxe2x88x921 or higher and a nitrous oxide capacity of 79 mmol/g/atm or higher at 30xc2x0 C. and the first, second and third adsorberts are optionally the same material.
The apparatus preferably further comprises in fluid series connection a cryogenic air separation unit.
In a third aspect, the present invention relates to a process for removing nitrous oxide from a feed gas stream, comprising passing the feed gas stream over an adsorbent having a nitrogen diffusion parameter of 0.12 secxe2x88x921 or higher and a nitrous oxide capacity of 79 mmol/g/atm or higher at 30xc2x0 C.